This invention relates in general to electrophotographic imaging members and, more specifically, to layered photoreceptor structures with overcoatings containing hydrogen bonded materials and processes for making and using the photoreceptors.
Electrophotographic imaging members, i.e. photoreceptors, typically include a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is an insulator in the dark so that electric charges are retained on its surface. Upon exposure to light, the charge is dissipated.
A latent image is formed on the photoreceptor by first uniformly depositing an electric charge over the surface of the photoconductive layer by one of any suitable means well known in the art. The photoconductive layer functions as a charge storage capacitor with charge on its free surface and an equal charge of opposite polarity (the counter charge) on the conductive substrate. A light image is then projected onto the photoconductive layer. On those portions of the photoconductive layer that are exposed to light, the electric charge is conducted through the layer reducing the surface charge. The portions of the surface of the photoconductor not exposed to light retain their surface charge. The quantity of electric charge at any particular area of the photoconductive surface is inversely related to the illumination incident thereon, thus forming an electrostatic latent image.
The photodischarge of the photoconductive layer requires that the layer photogenerate conductive charge and transport this charge through the layer thereby neutralizing the charge on the surface. Two types of photoreceptor structures have been employed: multilayer structures wherein separate layers perform the functions of charge generation and charge transport, respectively, and single layer photoconductors which perform both functions. These layers are formed on an electrically conductive substrate and may include an optional charge blocking and an adhesive layer between the conductive layer and the photoconducting layer or layers. Additionally, the substrate may comprise a non-conducting mechanical support with a conductive surface. Other layers for providing special functions such as incoherent reflection of laser light, dot patterns for pictorial imaging or subbing layers to provide chemical sealing and/or a smooth coating surface may be optionally be employed.
One common type of photoreceptor is a multilayered device that comprises a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. The charge transport layer can contain an active aromatic diamine molecule, which enables charge transport, dissolved or molecularly dispersed in a film forming binder. This type of charge transport layer is described, for example in U.S. Pat. No. 4,265,990. The disclosures of this patent is incorporated herein in its entirety. Other charge transport molecules disclosed in the prior art include a variety of electron donor, aromatic amines, oxadiazoles, oxazoles, hydrazones and stilbenes for hole transport and electron acceptor molecules for electron transport. Another type of charge transport layer has been developed which utilizes a charge transporting polymer wherein the charge transporting moiety is incorporated in the polymer as a group pendant from the backbone of the polymer backbone or as a moiety in the backbone of the polymer. These types of charge transport polymers include materials such as poly(N-vinylcarbazole), polysilylenes, and others including those described, for example, in U.S. Pat. Nos. 4,618,551, 4,806,443, 4,806,444, 4,818,650, 4,935,487, and 4,956,440. The disclosures of these patents are incorporated herein in their entirety.
One of the design criteria for the selection of the photosensitive pigment for a charge generator layer and the charge transporting molecule for a transport layer is that, when light photons photogenerate holes in the pigment, the holes be efficiently injected into the charge transporting molecule in the transport layer. More specifically, the injection efficiency from the pigment to the transport layer should be high. A second design criterion is that the injected holes be transported across the charge transport layer in a short time; shorter than the time duration between the exposure and development stations in an imaging device. The transit time across the transport layer is determined by the charge carrier mobility in the transport layer. The charge carrier mobility is the velocity per unit field and has dimensions of cm2/volt sec. The charge carrier mobility is a function of the structure of the charge transporting moiety, the concentration of the charge transporting moiety in the transport layer and the electrically "inactive" binder polymer in which the charge transport molecule is dispersed (if the transport layer consists of charge transporting molecules dispersed in a binder). It is believed that the injection efficiency can be maximized by choosing a transporting moiety whose ionization potential is lower than that of the pigment (assuming the charge transporting carriers are holes). However, low ionization potential molecules may have other deficiencies, one of which is their instability in an atmosphere of corona effluents. A copy quality defect resulting from the chemical interaction of the surface of the transport layer with corona effluents is referred to as "parking deletion" and is described in detail below.
Photoreceptors are cycled many thousands of times in automatic copiers, duplicators and printers. This cycling causes degradation of the imaging properties of photoreceptors, particularly multilayered organic photoconductors which utilize organic film forming polymers and small molecule low ionization donor material in the charge transport layers. Such wear is accelerated when the photoreceptor is utilized in systems employing abrasive development systems such as single component development systems. Wear is an even greater problem where a drum is utilized which has such a small diameter that it must rotate many, many times merely to form images for each conventional size 8.5 inch by 11 inch document. Wear of the photoreceptor can be compensated by increasing the thickness of the charge transport layer. However, large increases in thickness of the charge transport layer can render the photoreceptor inoperable at high imaging process speeds because of the very long transit times of common charge transport layer materials. Also, large decreases in thickness due to wear can cause dramatic changes in electrical characteristics in only a few thousand cycles that cannot be readily compensated by even sophisticated computerized control apparatus.
When the electrophotographic imaging member is utilized in liquid ink development systems, leaching of small molecules from the charge transport layer into the liquid development can occur. Loss of the small molecule material due to leaching causes undesirable deterioration in electrical properties of the photoreceptor. Also, undesirable crystallization of the small molecule in the charge transport layer can adversely affect the electrical imaging characteristics of the photoreceptor.
Reprographic machines utilizing multilayered organic photoconductors also employ corotrons or scorotrons to charge the photoconductors prior to imagewise exposure. During the operating lifetime of these photoconductors they are subjected to corona effluents which include ozone, various oxides of nitrogen, etc. It is believed that some of these oxides of nitrogen are converted to nitric acid in the presence of water molecules present in the ambient operating atmosphere. The top surface of the photoconductor is exposed to the nitric acid during operation of the machine and charge transport moieties at the very top surface of the transport layer are converted to what is believed to be the nitrated species of the molecules. It is believed that the chemical transition state species could form an electrically conductive film. However, during operation of the machine, the cleaning subsystem continuously removes (by wear) a region of the top surface thereby preventing accumulation of the conductive species. Unfortunately, such is not the case when the machine is not operating (i.e. in idle mode) between two large copy runs. During the idle mode between long copy runs, a specific segment of the photoreceptor comes to rest (is parked) beneath a corotron that had been in operation during the long copy run. Although the high voltage to the corotron is turned off during the time period when the photoreceptor is parked, some effluents (e.g. nitric acid, etc.) continue to be emitted from the corotron shield, corotron housing, etc. This effluent emission is concentrated in the region of the stationary photoreceptor parked directly underneath the corotron. The effluents render that surface region electrically conductive. When machine operation is resumed for the next copy run, image spreading and loss of resolution occurs in the region of the photoconductor where surface conductivity was increased. Deletion may also be observed in the loss of fine lines and details in the final print. Thus, the corona induced changes primarily occur at the surface region of the charge transport layer. These changes are manifested in the form of increased conductivity which results in loss of resolution of the final toner images. In the case of severe increases in conductivity, there can be regions of severe deletions in the images. This problem is particularly severe in devices employing arylamine charge transport molecules such as N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine and charge transport polymers incorporating diamine transporting moiety.
Although, "parking deletion" is described above, in some cases deletion might occur in all portions of the photoconductor. This will depend on the number and type of corotrons employed, the design of the photoconductor cavity and air-flow patterns around the photoconductor.
Thus, although the charge transport moiety or species meets most other electrophotographic criteria such as being devoid of traps, having high injection efficiency from many pigments, ease in synthesizing, and inexpensive, it encounters serious parking and other deletion problems when an idle mode is interposed between extended cycling runs.